It is known that photophysiological processes are crucial for plant growth and that these processes are controlled by photopigments, which exhibit strongly wavelength dependent absorption spectra. Chlorophylls, for example absorb light in the wavelength range of approximately 620 nm to 680 nm, whereas different forms of phytochrome, Phytochrome reduced (Pr) and Phytochrome far-red (Pfr), which regulate a range of molecular and physiological responses in plants, react to light within wavelength ranges centered in the red and far-red, respectively. The ratio, for example, between the absorption by Pr and Pfr controls physiological processes in plants like blossom induction, stem stretching, germination etc.
To improve the spectral profile of artificial illumination of plants, fluorescent lamps, which are commonly low-pressure mercury vapor discharge lamps with phosphor coating, can be used. These lamps, however, disclose a number of drawbacks such as limited efficiency, contain mercury which is a hazardous agent, have short lifetime, are fragile, require high voltage, and emit unwanted infrared light.
Alternatively, are today solid state light sources such as Light Emitting Diodes (LEDs) also used for plant illumination, since LEDs provide longer lifetime, higher photon flux efficacy, lower operating voltage, narrow-band light emission, and flexibility in terms of assembly compared to conventional light sources.
WO 2010/053341 discloses a phosphor converted LED for plant cultivation comprising a semiconductor chip, generating short-wavelength (blue or near-UV) light, and a wavelength converter containing at least one phosphor, converting the said short-wavelength light to longer-wavelength light due to photoluminescence. The longer-wavelength light contains a far-red spectral component peaking in the spectral range of about 700 nm to 760 nm, corresponding to the absorption spectrum of Pfr.
To this end, far-red light can also be achieved by using direct far-red LEDs, which are for instance based on AlGaAs or AlInGaP semiconductor materials.
There are, however, problems related to prior art; far-red light generating LEDs using phosphor conversion from blue light are, for instance, due to their inherent large Stokes shift, not efficient. The direct far-red LEDs are also relatively inefficient and furthermore not widely available.
There is, therefore, a need for abundant artificial light sources providing more efficient narrow wavelength band illumination during plant cultivation.